Your furnace utilizes various components that work together to provide cozy warmth throughout your home during the colder months. Therefore, when one or more of the furnace's components are faulty, it can result in the furnace blowing cold air.

man freezing inside his office

Some of the most common issues that result in a furnace blowing cold air include:

Cause #1: The Thermostat Is Set on the Wrong Setting

If your furnace continuously runs or the unit only blows out cool air sometimes, it could simply be that your thermostat is on the wrong setting.

Solution

Turn the thermostat setting to AUTO. When the fan setting is set to ON, it causes the furnace to blow out air even though the furnace is not producing heat. This can result in the furnace blowing cold air.
However, if you set the thermostat to AUTO, it will only blow out warm air.

Cause #2: Faulty Pilot Light

A faulty pilot light, including a pilot light that doesn't come on or a pilot light that doesn't stay on, can also cause your furnace to blow out cool air.

Solution

First, relight the pilot light and see if if remains on. If not, check the gas flow to the furnace to ensure it is receiving fuel.
Next, check that the furnace's gas valve switch is on and that gas is flowing into the furnace.
If the pilot light is dirty, it can also obstruct fuel from flowing through the furnace. Consequently, if the pilot light is dirty, clean it to restore proper gas flow.
Grime build-up on the burner can also cause the pilot light to have problems lighting, so have your burner cleaned to see if it helps.
A faulty gas valve can also cause your pilot light not to stay lit. If it has been a while since your furnace has been cleaned, it can accumulate dirt and debris, which can cause your valve to stick and prevent your pilot light from coming on.
Issues with the thermocouple, which is a sensor that manages the ignition as well as the gas valve, can also cause your pilot light not to stay lit. So if you have a faulty thermocouple, be sure to adjust or replace it to restore heat to your furnace.

Cause #3: Faulty Electronic Ignition

Newer furnaces utilize an electronic ignition, which can cause problems if it is not adjusted properly.

Solution

Inspect the electronic ignition, and then have it adjusted or replaced if needed to restore proper function.

Cause #4: The Filter Is too Dirty

A dirty filter obstructs airflow over the unit's heat exchanger. This causes the furnace to run longer until eventually, it runs hot.

When the furnace gets too hot, it trips the high-temperature limit switch. This forces the burners to shut off, so the heat exchanger doesn't get damaged. However, the blower continues to blow to keep the furnace at a safe level, which can result in the furnace blowing cold air.

If you notice that the furnace blows out warm air and then cool air, or it stops blowing out air altogether after a while, it could be that a dirty filter is causing the furnace to overheat.

Solution

Shut off power to the furnace, and then inspect the filter. If the filter is dirty, then clean it or replace it, and then reset the system.

Cause #5: The Condensate Line Is Blocked

If your furnace is a newer high-efficiency model, and you notice water flooding around the unit, it could be due to a blocked condensate line.

When the condensate line is blocked, typically by dirt, dust, mold, or ice, it causes water to back up into the unit. This causes the furnace switch to shut off to prevent water damage.

A broken condensate pump can also cause the condensate line to overflow.

Solution

To unclog the condensate line, first shut off the power to your HVAC system.
Locate the drip pan on the interior air handler unit.
Use towels or a wet dry vac to remove moisture from the pan.
Wash away any contamination from the drip pan with soap.
Use the wet-dry vac to clear the block from the drain line. If the vacuum does not clear the clog, try running a pliable rubber tube through the line to clear the clog.
You also need to clean the drain line at the entry point. Remove the PVC cover, and use hot water with a mild soap to clean out the drain. You can also use distilled vinegar to clean out the drain.
Allow the soapy water or vinegar to sit for about a half hour. Afterward, rinse the line with fresh water, ensuring water flushes through the entire line freely.
If the condensate line is clogged by ice, try covering the line with pipe insulation and heat tape to prevent the contents from freezing.

Cause #6: A Leak or Clog in the Ductwork

Cracks, holes, and gaps in the ducts allow cold air from the attic or crawlspace to enter into the ducts, which can make it feel like your furnace is blowing out cool air.

Dirt and debris accumulated on the inside of the ducts can also hinder airflow.

Solution

Inspect the ducts in the attic or crawlspace and around the unit for cracks, leaks, and gaps in the ductwork. Afterward, have a professional technician repair any openings if needed.

Summing Up

A furnace blowing cold air can be caused by various issues with your furnace's components, many of which you can address yourself. However, for more serious issues with your furnace blowing cold air, contact a technician who is highly knowledgeable and skilled in the inspection and repair of a furnace blowing cold air to prevent damaging your system.